In general, the term judder means to vibrate with intensity. In the field of displaying images, the term judder generally means a visual artifact arising from dropped fields or frames and having the appearance of jerky or stuttering camera movement. Judder may be introduced when images are sampled at a first frame rate and then converted to a second frame rate for viewing purposes. Additional frame rate conversions may further increase the presence of judder.
Typically, multiple frame rate conversions are not required when displaying most images. Usually, a single frame rate conversion may be sufficient. An image sequence at a first frame rate being frame rate converted to a second frame rate, which is the frame rate of an image display device.
One type of image display where multiple frame rate conversions may commonly appear is image overlay. Image overlays are images formed when at least two image sequences with potentially different frame rates combined into a single image sequence and then displayed on an image display device. FIG. 1a displays an image overlay 100. Image overlay 100 comprises an image from a first image source 105 overlaying an image from a second image source 110. Image overlay 100 may commonly be referred to as a picture in a picture (PIP). Other forms of image overlay may be split screen, picture out of picture (POP), and so forth, generated with images from two, three, four or more image sources.
If the two (or more) image sequences have different frame rates, then a first image sequence (an overlay image sequence) may need to be frame rate converted to a frame rate of a second sequence (a primary image sequence) prior to the overlaying of images from the first image sequence onto images from the second sequence. Then, if a display device being used to display the images from the overlaid sequence uses a frame rate different from the frame rate of the second sequence, the overlaid sequence may also need to be frame rate converted prior to being displayed.
FIG. 1b displays a portion of a prior art video processor 150 used to overlay images from multiple image sequences. As shown in FIG. 1b, video processor 150 may overlay images from video input one onto images from video input two. Video processor 150 includes a video input one that provides a first sequence of images at a frame rate Frequency A (FREQ A) and a video input two that provides a second sequence of images at a frame rate Frequency B (FREQ B), where FREQ A is not equal to FREQ B.
Since the frame rates of the sequences of images do not match, the overlay images may not be created by simply combining the images from the first sequence and the images from the second sequence. The frame rate of one of the sequences may need to be converted so that the two frame rates match. Video processor 150 includes a first frame rate converter 155 coupled to the video input one. First frame rate converter 155 may perform a frame rate conversion on the first image sequence so that the frame rate of the first image sequence is substantially equal to the frame rate of the second image sequence.
The frame rate conversion performed by first frame rate converter 155 may require the repeating of some of the images in the first image sequence if the FREQ A is less than the FREQ B or the dropping of some of the images of the first image sequence if the FREQ A is greater than the FREQ B. In general, the dropping of images may cause the judder since image information is typically lost when images are dropped. This is especially true when there is motion present in the images being dropped. The repeating of images may not produce as many visual artifacts as the dropping of images since image information is generally not lost while duplicating images.
After matching the frame rate of the first sequence to the frame rate of the second sequence, an overlay control unit 160 may be used to create an overlay image sequence. The overlay image sequence will have a frame rate equal to the second sequence and if the frame rate of the display device is different from the frame rate of the overlay image sequence, then an additional frame rate conversion may be needed. A second frame rate converter 165 may be used to convert the frame rate of the overlay image sequence (at FREQ B) to the frame rate of the display device, FREQ C. The output of second frame rate converter 165 may then be provided to the display device, where the output will be displayed.
The frame rate conversion performed by second frame rate converter 165 may exacerbate any judder created in the frame rate conversion performed by first frame rate converter 155. The judder may be further exaggerated if the frame rate conversion performed by the second frame rate converter 165 results in the dropping of overlay images containing non-repeated images from the first image sequence while those containing repeated images are kept, since the dropping of the non-repeated images may further amplify the loss of motion information.
Additionally, the type of images in the image sequences used to create overlay images may have an impact on the visual artifacts. With sequences of interlaced images, wherein each interlaced image comprises an odd image field and an even image field, the dropping of image fields may result in more readily noticeable visual artifacts if more image fields of one type are being dropped. For example, if a number of consecutive even image fields (or odd image fields) are being dropped, the visual artifacts may be more noticeable.